My ex-husband lives in a one-bedroom house. During the school year, the boys are there on alternate weekends. This summer however, they will stay with him, and I will get them alternate weekends.

-His girlfriend stays over at his house, sometimes when the boys are there.
-We do not have a stipulation in our divorce papers about unrelated houseguests.

I asked him to please partition a 2nd bedroom for the boys in the house before June. He refuses to do this

Is there any law or precedent case in Michigan that addresses the number of occupants per bedroom, divorce or custody related?

To find out just how many people can occupy such a small house I would suggest you call up the Zoning Department of where this house is and ask them what the arrangements could be. You don't have to spell out just who lives where or the exact address, but how many people, how big/small a house, how many bedrooms, and you will get the official answer for that particular county's zoning regulations.

Where I live in Ohio that particular arrangement would not be allowed. Too many people, too small a house. It would not be allowed for more than 1 or 2 nights and certainly not for the entire summer.